1. Field of the Invention
The present invention relates to a method for reproducing information stored in an optomagnetic recording medium on which information is magnetically recorded and a device for carrying out the same method.
2. Related Background art
Recently, extensive studies and experiments of optical memory elements utilizing laser beams in order to obtain high data packaging density and large capacity memories or storages have been increasingly conducted. Among them, optomagnetic recording processes have been receiving increasing attention as a rewritable recording process and optomagnetic recording media used in such optomagnetic recording processes are greatly expected as rewritable optical memory or storage elements.
In general, with an optomagnetic recording medium the recording, reproduction and erasure are carried out in the manner described below.
In the case of the information recording mode, a recording medium is magnetized in a predetermined direction vertical to a substrate and then the laser beam spot which is modulated by the recording signal is projected on the recording medium. The medium can be magnetized in any predetermined direction. The spot of the medium, at which the laser beam is incident absorbs the light and its temperature is locally raised. When the temperature at this spot reaches about a Curie point of the material of the medium, an auxiliary magnetic field in the direction opposite to the direction of initial magnetization is applied so that the direction of magnetization at this spot is reversed, so that the reversed magnetic domain whose direction of magnetization is opposite to other portions impinged upon by with the laser beam is defined, whereby information is stored.
In the erasure mode, the information recorded spot is continuously illuminated with the laser beam so that the temperature of the recorded spot rises closer to a Curie point and then the magnetic field in the direction opposite to the direction of magnetization in the recording mode is applied, so that the recorded spot is returned to the state in which the magnetization direction of this spot is that of initial magnetization.
In the case of reading mode, the continuous laser beam whose power is decreased illuminates the recording medium, thereby reading out the direction of magnetization of recorded spots by utilizing the magnetic Kerr effect.
In this case, there arises the problem how to determine the level of the reproducing laser beam. The quantity of the reproducing laser beam must be so selected that magnetization will not be reversed in direction due to the temperature rise of the recording medium resulting from the illumination of the laser beam. However, if the quantity of the reproducing laser beam is too small, the quantity of the detected light is decreased, so that it becomes impossible to increase the S/N ratio of the reproduced signal. It follows therefore that the quantity of the reproducing laser beam should be slightly smaller than that (threshold quantity of light) at which the reversal of the direction of magnetization occurs.
However, the above-described threshold quantity of light cannot be uniquely determined because of variations in sensitivity of individual recording medium, aging of an sensitivity, environmental temperatures and so on. Therefore, in the prior art devices, variations in threshold quantity of light are taken into consideration and a quantity of light far smaller than a threshold is used in the reproduction or reading of information, so that the S/N ratio of the reproduced signal cannot be increased.